Electrically assisted printing system

ABSTRACT

A printing system having an electric circuit supplying an output potential to assist transfer of ink to a dielectric substrate, the potential being gradually increased up to a breakdown potential of the substrate, for example of a rate of 10 percent per hour. Each time breakdown potential is reached, the applied potential is set back 10 percent and the increase started again, so that the applied potential will follow variations in the breakdown potential of the substrate. A severe fault in the substrate results in removal of the output potential during a timed interval followed by a rapid potential buildup toward breakdown potential, after which the normal cycle is resumed.

United States Patent [72] Inventor Daniel A. Coberley Danville, Ill.[21] Appl. No. 853,672 [22] Filed Aug. 28, I969 [45] Patented Nov. 9,1971 [73] Assignee l'lurletron Incorporated Danville, Ill.

[54] ELECTRICALLY ASSISTED PRINTING SYSTEM 1 Claim, 4 Drawing Figs.

[52] U.S.Cl 317/3, 10l/153,l0l/17O [51] Int. Cl B41i9/06, B4Im 5/20 [50]Field of Search 101/150, 153,426,170;3l7/3,262,149

[56] References Cited UNITED STATES PATENTS 2,767,359 10/1956 Larsen etal. 317/3 3,295,441 1/1967 Garnier 3,477,369 11/1969 Adamsonetal.

ABSTRACT: A printing system having an electric circuit supplying anoutput potential to assist transfer of ink to a dielec tric substrate,the potential being gradually increased up to a breakdown potential ofthe substrate, for example ofa rate of 10 percent per hour. Each timebreakdown potential is reached, the applied potential is set back 10percent and the increase started again, so that the applied potentialwill follow variations in the breakdown potential of the substrate. Asevere fault in the substrate results in removal of the output potentialduring a timed interval followed by a rapid potential buildup towardbreakdown potential, after which the normal cycle is resumed.

1 ELECTRICALLY ASSISTED PRINTING SYSTEM SUMMARY OF THE INVENTION Thisinvention relates to an electric printing system and method andparticularly to such a system wherein an applied electric potentialacross a moving dielectric substrate assists in transfer of ink to thesubstrate.

An object of the invention is to provide an improved electrical printingsystem and method.

Another object is to provide an improved electric circuit formaintaining substantially an optimum applied electric potential forassisting in the transfer of ink to a dielectric substrate.

A further object of the invention is to provide an electric printingsystem wherein the applied potential is maintained near the breakdownpotential of the substrate in spite of any variations thereof duringnormal operation.

A feature of the invention resides in the provision of an electriccircuit for generally increasing the applied potential in an electricprinting system up to a breakdown potential value, the applied potentialbeing set back a predetermined amount each time the breakdown potentialis approached so that the applied potential essentially followsvariations in the dielectric strength of the printing medium duringnormal operation.

A subsidiary feature resides in the provision of a start cycle whereinthe potential applied by said electric circuit is initially increased ata substantially higher rate so as to relatively rapidly approach theoptimum operating range.

Another subsidiary feature resides in the provision of means forremoving the output potential in response to a serious fault in theprinting substrate and then increasing the applied potential at arelatively rapid rate after a predetermined time delay.

Other objects, features and advantages of the invention will be readilyapparent from tee following description of a preferred embodimentthereof, taken in conjunction with the accompanying drawings althoughvariations and modifications may be effected without departing from thespirit and scope of the novel concepts of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1a and lb together illustrate apreferred electric circuit in accordance with the present invention.FIG. lb being a continuation of the circuit of FIG. la to the right;

FIG. 2 illustrates an exemplary operating sequence for the electriccircuit of FIGS. 1a and lb and specifically represents the magnitude ofthe negative potential across the capacitor C21 ofFlG. lb; and

FIG. 3 illustrates the output electric potential variation for thesequence of operation illustrated in FIG. 2, the time scale in FIG. 3corresponding to the time scale in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. lb,the electric circuit has been illustrated as applied to a gravure-typeprinting system including a gravure cylinder 10, an impression roller 11and a dielectric substrate 12 moving in the direction of the arrow 13through a printing nip between the gravure cylinder and impressionroller. In a commercial printing system of this type, the gravurecylinder may be of electrically conductive metal, while the impressionroller 11 may have a metal core with a layer of insulating rubber and anouter covering of semiconductive rubber to which an electric potentialis applied by means of a conductive roller or the like as represented bythe contact 14. The applied potential produces an electric current flowin the covering of semiconductive rubber so as to continuously supplyelectrical energy at the nip region indicated at 15 between the gravurecylinder 10 and impression 11. Typically sufficient downward force isapplied by the impression roller 11 so as to result in a force of50 to100 pounds for each lineal inch along the length of the nip region 15(at right angles to the direction of travel of the substrate 12). In theillustrated embodiment, a direct current potential is applied, but it iscontemplated that an alternating current potential may also be employed.The invention is not limited to a gravure printing system but isapplicable also to other types of printing.

Referring to the left hand side of FIG. la, a switch S1 suppliescommercial 60-hertz electric power to the system, energizing a fan motor20 and a neon power" indicator N1. The input is isolated by inductorsLIA and LIB and capacitors C17 and C18 to prevent transients fromtriggering a crowbar cycle (to be hereinafter described).

Depressing the start button closes contacts Start A" shown at the leftcenter of FIG. 1a and also closes the contacts Starts B shown at thelower right of FIG. 1b. Energization of relay K6 simulates momentarypressing of the start button each time the external press go-down"circuit is completed between contacts 21 and 22 at the lower left ofFIG. In. Each time the circuit between contacts 21 ad 22 is completedindicating that the printing substrate 12 is moving at proper speed,relay K6 is momentarily actuated as capacitor C20 is charged. When thecapacitor C20 reaches a predetermined charge, relay K6 is thendeenergized releasing the contacts K6-A and K6-B which parallel the Aand B start contacts. A holding circuit for relay K3 exists throughcontacts S4-D, S2-A, the stop switch contacts and contacts K3-A andthrough S3-A and through the go-down circuit between contacts 21 and 22.

Relay K4 connects the output at 24 in FIG. lb to the press (specificallyto the impression 11 and printing cylinder 10) via normally open contactK4-2 of relay K4. It will be noted that by reversing selector switch S4,contacts 84-8 and S4-C will connect the positive output conductor 24with the grounded gravure cylinder 10, rather than to the impressionroller 11 as in the illustrated position of the selector switch S4. Whenrelay K4 is deenergized, the normally closed contacts 1(4-1 at the upperright of FIG. lb connect the output of the dummy load resistors R33-R39.Relay K4 can only be energized if the mode switch S3 is actuated to theoperate position, closing contact S3-A shown at the lower left in FIG.10.

The setup relay K3 has contacts K3-C as indicated in the central part ofFIG. 1a which must be open to allow operation of the series regulatorincluding transistors 02. Q5 and Q6. Either one of two setups may latchK3. In "test," the mode switch S3 is in test position and the "stop"button is latched in the lower position to energize relay K3 throughcontact 83-81 and stop contacts of the stop button. In the operate"position of the mode switch S3 the external godown" circuit between 21and 22 is completed and either the start button is momentarily depressedor the go-down" setup relay K6 is momentarily energized.

The range switch S2 and the polarity switch 84 each have a pole (S2-Aand S4-D) in series with the setup relay K3 to unlatch the relay at eachnew selection.

Relay K2 whose energizing coil is indicated at 25 at the lower right inFIG. lb is the filament relay. The K2 coil 25 is in series with thefilament circuit of the thyratron Q16 shown at the lower right in FIG.lb, and will be energized only if the filament circuit draws current.Relay K2 has a single set of contacts K2 which in the normally closedcondition holds the series regulator 02, Q5, O6 in the off conditionwhen relay K2 is deenergized.

Current to the high-voltage generator indicated generally at 30 (at theupper part of FIG. lb) is supplied via line 31 and is controlled by theinput to transistor 02 of regulator 32 (the upper right of FIG. la).Zener diode D10 and resistors R13 and R14 control the supply ofaconstant current to the regulator 32, a comparison amplifier provided bytransistor Q4, and a reference diode D6. Feedback voltage from resistordivider R30, R31, R32, P7 and P6 (at the upper right of FIG. 1b) issupplied via conductor 34 to the input of the comparison amplifier Q4and is compared with the reference voltage across diode D6. As feedbackis increased. transistor 04 bypasses a larger portion of the constantcurrent away form the series regulator 32. Thus the regulator outputdecreases the suhsequent voltage across the resistor divider untilstabilization is reached. Hence adjustment of the resistor divider ratioby means of S2-B, P6 and P7 governs the output voltage at output 24.Capacitor C4 and resistor R6 form an RC ramp circuit to limit the turnon rise time of the regulator. If any of the relay contacts K2, 1(3-Cand Kla are closed, the capacitor C4 will be held to essentially a zerovoltage thereacross. Diode D4 will clamp the input of series regulator32 to the voltage across capacitor C4 and thus will hold the regulator32 to the voltage across capacitor C4 and thus will hold the regulator32 open when any of the contacts K2, K3-C and Kla are closed. If all ofthese relay contacts are open, capacitor C4 will charge and the clampingdiode D4 will follow and bring the regulator 32 along. When theregulator reaches stabilization, the capacitor C4 will continue tocharge and will reverse bias diode D4.

Transistors Q15 and Q11 at the center part of FIG. 1b form an oscillatorwith the associated passive components. The oscillator is an astablemultivibrator running at approximately 1,000 hertz. Potentiometer P2serves as a symmetry balance. The oscillator is constructed of PNPtransistors and operates from the negative supply to achieve low outputimpedance to drive bistable multivibrator 36. The oscillator runscontinuously.

Bistable multivibrator 36 uses transistors Q9, Q10, Q13 and Q14, anddrives transformer T3 with a square wave of varying amplitude dependenton the regulator input power to the bistable via conductor 31. Emittercurrent passes through a bimetal breaker N10 via conductor 37, thecomponent N10 appearing at the right center of FIG. 1a and servingtoprevent excessive generator current. The voltage drop across thebreaker when the breaker opens is supplied via resistor R8 and conductor38 to a safety circuit including capacitor C3, neon tube N5 and siliconcontrolled rectifier Q1. Thus circuit operates to remove the outputpotential from output conductor 24 for a predetermined time intervalwhen the circuit breaker opens. As will hereinafter be described, thesame circuit averages crowbar pulses by means of capacitor C3 and tripsthe time delay at a preset average. The time delay circuit includingtransistor Q12 and silicon controlled rectifier Q8 at the lower right ofFIG. la delays the supply of power to the high voltage generator eachtime the system is turned on, and each time the safety circuit istripped. The crowbar circuit including the thyratron 016 at the lowerright of FIG. lb responds to excess current flow in potentiometer P4 atthe center part of FIG. lb to render the thyratron Q16 conductive inresponse to output current flow in excess of normal in the system so asto immediately short circuit the output 24 via conductor 40, thyratronQ16 and conductors 41 and 42. This discharges the press components 10and 11 and prevents any further power from being delivered thereto. Itinterrupts the power supply to the press for the time that the webdefect is in the printing nip 15. The amount of current rise which willtrigger the crowbar circuit is adjustable by means of the current tripset control which controls potentiometer P4 associated with crowbarfeedback conductor 44 at the center of FIG. 1b.

Transformer T1 at the center of FIG. 10 has two secondary windingssupplying rectifier bridges D1 and D2. Bridge D1 together with resistorR1 and capacitor C1 supplied approximately 3 amperes at plus 50 voltsdirect current to the regulator 32 via conductor 46. Bridge D2 togetherwith resistor R2 and capacitor C2 supplies approximately one-half ampereat minus 50 volts direct current to conductor 47.

The maximum output power of the transformer T3 at the upper center ofFIG. 1b is approximately 7 kilovolts at 7 milliamperes. Rectifiers D12,D13, D14 and D15 and capacitor C14 convert the power to direct current.Resistor R29 serves as a plate load resistor for thyratron Q16 duringthe crowbar function and serves as a limit resistor during regularoperation to isolate capacitor C14 from the output. Diodes D17 and D18clamp the output conductor 24 of the high-voltage generator to thereturn line 50 when no power is being applied to the press components 10and 11.

Potentiometers P4 and P5 connect return conductor 50 to the negativeside of the high-voltage generator 30. Current drawn by the presscomponents 10 and 11 will appear as a negative voltage across thepotentiometers. As previously mentioned, a portion of this voltage isfed back by means of conductor 44 to the crowbar input circuit at thebase of transistor Q3.

The crowbar circuit consists of transistors Q3 and Q7, relay Kl (at thelower right of FIG. 1a and the thyratron Q16. Transistor Q3 which formsthe first stage of the crowbar circuit is an emitter follower, so thatthe output voltage essentially equals the input voltage. Diode D7, FIG.1a which is connected to the emitter of Q3 insures that the transistorQ7 will turn full off when the signal from O3 is removed. When presscurrent increases, diode D7 will be forward biased and transistor 07will conduct an amount preset by potentiometer P4.Resistor R16 andCapacitor C8 in the base circuit of Q7 average the output of transistorQ7 by the Miller effect. If transistor Q7 is off (nonconducting), novoltage will be developed across the relay energizing coil of relay K1.Hence, the thyratron Q16 is held off by the negative supply through therelay coil of K1. If for example an arc occurs between press components10 and 11, transistor Q7 turns on very quickly. The coil of relay Klappears as a high impedance and the grid voltage of thyratron Q16 isbypassed to the common return line 50, thus allowing the thyratron tofire in approximately 10 microseconds. The thyratron shorts across thepress components 10 and 11 and applies a heavy load to the highvoltagegenerator 30. Hence more current flows in potentiometer P4 and resultsin saturation of transistor 07 until relay K1 has energized (inapproximately 5 milliseconds).

Relay Kl has snap action single pole double throw contacts Kla and Klbshown at the center of FIG. la. The normally open contacts Kla are inparallel with contacts K2 and K3-C to also turn on and off the regulator32. Energization ofthe Kl relay turns off the regulator 32, thusremoving power to the high-voltage generator and allowing the thyratronQ16 to reset. Since current flow in potentiometer P4 has ceased, theinput signal at conductor 44 at the input to the crowbar amplifier isremoved, and the crowbar relay K1 is deenergized. This then completesone cycle of the crowbar circuit.

The safety circuit involves silicon-controlled rectifier Q1, triggerneon N5, capacitors C3 and C6, and the normally closed contacts Klb ofthe K1 relay. Each time the crowbar circuit cycles, the Klb contactsopen and close to actuate the fault indicator N8 and to supply anegative-going square wave to capacitor C6. Capacitor C6 produces anegative going pulse at conductor 38 in response to the leading edge ofthe square wave and supplies a positive pulse in response to thetrailing edge. The negative pulse is routed via conductor 38, resistorR40 and diode D27 to actuate transistor Q17. The positive pulse issupplied via diode D3 and potentiometer P1 to capacitor C3. Theaveraging of the charge supplied to capacitor C3 is adjusted by means ofthe potentiometer P1. If the average of the positive pulses supplied tocapacitor C3 is high enough, the neon N5 breaks over and fires thesafety silicon-controlled rectifier Q1.

The safety silicon-controlled rectifier Ql performs the followingfunctions: one, it fires the safety indicator N9; two, it opens theregulator 32 by discharging capacitor C4 through resistor R4; and three,it commutates the time delay siliconcontrolled rectifier Q8 throughcommutating capacitor C5.

The time delay circuit uses a unijunction transistor Q12, a time delaysilicon-controlled rectifier Q8 and crowbar relay Kl. The base tworeference and charging voltage to capacitor C22 are both taken from theanode of Q8. If O8 is nonconducting, diode D8 will be forward biased andenergize the crowbar relay Kl from common conductor 50 through rcsistorR18. The voltage drop across the relay K1 will serve as the inputvoltage to the time delay transistor 012. When 08 is conducting throughresistor R18, diode D8 is reversed biased and the relay K1 is under thecontrol of crowbar transistor 07. When 08 is conducting, it commutatesO1 to the nonconducting condition by means of capacitor C5, and itcommutates the automatic advance silicon-controlled rectifier 019 at thelower left of FIG. lb to the nonconducting condition by means ofcapacitor C19 at the lower left of FIG. lb. The shut down of seriesregulator 32 on each safety trip has redundant control since Q1 turnsoff the regulator via resistor R4 while 08 turns off the regulatorthrough relay K1, thus doubly insuring shut down in case of componentfailure.

In the automatic mode, an automatic voltage adjustment transistor 020 atthe upper left of FIG. lb and a rheostat P9 are placed across thefeedback potentiometer P7 and trimmer potentiometer P6, thepotentiometer P7 being fully counterclockwise so as to hold contactsP7U-A (at the lower right FIG. la) and contacts P7S-B (at the lowercenter of FIG. 1b) in the open condition as shown. Transistor Q is ajunction field effect transistor used as a variable resistor. Thispermits electronic adjustment of the voltage divider ratio which issupplied via conductor 34 to the input of the series regulator 32.Accordingly, control of the transistor Q20 will serve to control theoutput voltage at output conductor 24. The high input impedance of Q20and low leakage of capacitor C21 and diodes D21 (at the center left ofFIG. 1b), D24 and D29 (at the lower center of FIG. 1b) allow thecapacitor C2] to control the output voltage of the generator 30 overextended periods of time with little drift. The reverse current ofdiodes D31 (at the upper left of FIG. 1b) and D21 compensate thecoefficients of diodes D24 and D29, respectively. A small negativeoffset voltage is utilized from resistor R51 and diode D26 to insuresufficient turn on of transistor Q20. The output voltage at conductor 24is inversely proportional to the charge on capacitor C21, consequentlyleakage from capacitor C21 results in a long term increase in the outputvoltage at conductor 24 during automatic operation.

Capacitor C21 acquires a charge from two independent circuits. If theunit is in manual mode, capacitor C21 is charged to the voltage of zenerdiode D30 (at the lower center of FIG. lb) from conductor 47 throughR52, contacts KS-C and conductor 60. Relay K5 may be energized byactuation of potentiometer P7 to momentarily close contacts P7S-B.

If the unit is in automatic mode and is recycled, the turn off of Q8provides a relatively high input potential at the base of Q18 (fromcommon conductor 50 via R18, conductor 61, R41, D19 and R45) so as torender Q18 conducting for the duration of the timing cycle, allowingcharging of capacitor C21 from the negative conductor 47 through Q18,R47, D21 and conductor 60.

The automatic setback amplifier Q17 at the center left of FIG. 1bamplifies each negative pulse received from the crowbar relay contactsKlb via capacitor C6, and each pulse passes on to the automaticswitching transistor Q18 which will conduct and allow charging ofcapacitor C21 for the duration of each such negative pulse to amplifierQ17. By way of example, the charge supplied to capacitor C6 may reducethe voltage to output conductor 24 by approximately 10 percent. Thus inresponse to each crowbar cycle, capacitor C21 receives an increment ofcharge sufficient to reduce the output voltage from the circuit by about10 percent. When, however, the safety circuit is actuated by capacitorC3, capacitor C21 is recharged to a voltage determined by zener diodeD30, which charge corresponds to a selected lowest operating outputvoltage at conductor 24 (after the safety cycle has been completed andthyratron Q16 reset).

The switching transistor Q18 is held in a conducting state if the timedelay silicon-control rectifier O8 is nonconducting through resistor R41and diode D19 during a safety cycle as previously described. The diodeD19 insures that the forward drop when the time delay silicon-controlledrectifier O8 is conducting will not hold transistor Q18 in theconducting condition. Diode D21 is an isolation diode. The switchingtransistor Q18 causes charging of capacitor C21 from the minus 50 voltsupply conductor 47 for improved linearity. The clamping diode D29 willlimit the charge on capacitor C21 to the zener voltage of D30.

At the end of a safety cycle, after capacitor C21 is fully charged, theautomatic advance silicon-controlled rectifier Q19 (at the lower left ofFIG. 1b) is rendered nonconducting, to permit capacitor C21 to dischargerelatively rapidly through diode D24 and resistor R49 to the positivesupply conductor 46. Diode D25 prevents the capacitor from chargingpositively by clamping the anode of diode D24 through resistor R53 andconductor 63 to the common conductor 50. When the automatic advancesilicon-controlled rectifier Q19 is conducting, diode D25 is forwardbiased and diode D24 is reverse biased. Thus, no discharging current canflow through diode D24 once a safety cycle has been completed. Each timethe time delay silicon-controlled rectifier Q8 fires, the automaticadvance silicon-controlled rectifier Q19 is commutated off throughcommutating capacitor C19 and diode D20, so as to permit the rapiddischarge of capacitor C21 and a corresponding relatively rapid increaseof the output potential at conductor 24 from the minimum operatingpotential up to a desired operating level which as will hereinafter beexplained will approach the dielectric breakdown strength of theprinting medium 12 for the illustrated embodiment. Crowbar pulses fromthe setback amplifier Q17 are fed to the gate of the silicon-controlledrectifier Q19 through resistor R42, so that the first crowbar pulseafter a safety cycle fires Q19 and stops the discharge of capacitor C21at the relatively rapid rate. Diode D20 prevents commutation from theautomatic advance SCR, Q19 back to the time delay SCR, Q8.

In accordance with the concepts of the present invention thehigh-voltage generator 30 is feedback controlled via conductor 34. Thefeedback voltage, however, is additionally controlled by the regulatingline 70 appearing at the top of FIG. 1b and leading to the transistorQ20 whose effective resistance is controlled by the charge on capacitorC21. The output of the charging unit at 24 is supplied to components 10and 11 to establish a current flow in the return circuit extending fromcomponent 10 via switch contact S4-B, conductor 71, conductor 72,inductor L2, ammeter Ml, return conductor 50, and potentiometers P5 andP4. As the substrate 12 has a dielectric strength defined in volts permil (1 mil equals 0.001 inch) thickness, the maximum potential that canbe applied between components 10 and 11 is limited by the dielectricstrength of the substrate 12. This factor will vary with thickness,relative humidity and moisture content of the substrate. Furthermore,the substrate under normal conditions is not perfect and does exhibitpin holes, minute variations in thickness and foreign particles thatresult in dielectric breakdown in the practical case prior to thedielectric breakdown of the perfect material. The dielectric breakdownof the substrate l2 physically ruptures the material and an are or sparkis created. This must be extinguished prior to the material leaving theink transfer zone 15. If it is not, a hazardous condition is establisheddue to the extremely hazardous (explosive) environment that inherentlyexists, for example in a gravure printing system. In accordance with thepresent invention, it is desired to establish a maximum electrostaticforce on the ink at the ink transfer region 15, and accordingly it isdesired to maintain the potential between press components 10 and 11 ata value near but not exceeding the dielectric strength of the material12.

A previously described, when the dielectric strength of the substrate isexceeded, a sharp spike of current is drawn which is sensed at crowbarfeedback conductor 44 as previously described. The current is developedby the discharge of the area of the impression roller 11 above thefault. The spike of current causes the thyratron 016 to ignite and shuntthe charging unit and the impression roller, so that the capacitorformed at the ink transfer zone is connected to ground in less thanmicroseconds. The circuit extends from the contact 14 of impressionroller 11 through contact S4C, conductor 74, contacts K4-2, conductor40, thyratron Q16, conductors 41 and 42 and contact $412 which in turnis connected to the metal of the impression roller 10 as indicated byconductor 75. This action removes energy from the ink transfer zone andextinguishes the are before the substrate 12 leaves the nip region 15.Due to the imperfect nature of the substrate these faults are considerednormal to the operation of the system.

Manual operation of the charging unit allows the operator to establishthe potential of the ink transfer zone using these faults as anindication of the optimum operating potential, faults being indicated byindicator N8, for example. Due to environmental conditions andvariations in substrates the optimum operating potential may changeafter a period of time and may either increase or decrease. Thus tocarry out manual operation would require the operator to constantlymonitor the equipment. As a fault established in the nip is nothazardous if it is contained within the ink transfer zone 15, it isconceived that such fault indications may be used to create an automaticsystem.

In the illustrated system, at start up or in a safety recycle operation,the output voltage at conductor 24 increases on a ramp function orlinearly until the dielectric strength of the substrate 12 is exceededand a fault or dielectric breakdown occurs. At this point, amplifier 018is rendered momentarily conducting, to supply an increment of charge tocapacitor C21 so as to reduce the output voltage of the unit so that theunit supplies approximately percent less voltage then the potential thatresulted in breakdown. A second ramp is then initiated that increasestheoutput potential at a much slower rate, for example by leakage ofcharge from capacitor C2] through the leakage resistance of diode D31 tothe positive conductor 46, FIG. lb. This discharge rate may be such thatthe output voltage at conductor 24 increases approximately 10 percentper hour. This second long ramp increases the voltage very slowly up tothe maximum dielectric strength of the web 12. If the dielectricstrength of the web has improved since the initial setting then thevoltage will slowly increase until a new limit is established. If,however, the dielectric strength of the web has decreased, a series offaults occur during a short period of time and a safety cycle ininitiated and after another period of time recharge capacitor C21 sothat the unit will turn on at a selected lowest operating potential,Utilizing this cycling process, the optimum voltage is automaticallymaintained at the nip without relying on a human operator and theconsequent possibility oferrors. In addition, it removes the possibilitythat in the over-voltage condition a hazard could be created in thepressroom.

Basically, what is done is to apply a potential high enough to result inthe dielectric breakdown of the substrate 12 to be printed. Thebreakdown is sensed by the increased current in the system, for example,and the applied potential is then reduced for example to about 90percent of the potential which produced the dielectric breakdown. Byslowly increasing the potential thereafter, the potential chases" thedielectric strength of the substrate. If a large imperfection occurs inthe substrate, the equipment entirely removes the potential from the inktransfer zone 15 and repeats the initial cycle.

SUMMARY OF OPERATION The operation of the illustrated embodiment may besummarized by referring to the operating sequence illustrated in FIGS. 2and 3. FIG. 2 represents the quantity of charge or value of negativepotential on capacitor C21, while FIG. 3 represents the correspondingoutput potential at conductor 24 relative to ground potential (asrepresented by conductor 75). FIGS. 2 and 3 are on a comparable timescale, but the illustration is purely diagrammatic and relative timeintervals are not proportionately represented on the time base of FIGS.2 and 3.

Referring to FIG. 2, successive operating cycles have been representedby the curve segments 81-89, while the corresponding output voltageshave been represented in FIG. 3 by segments 91-99, respectively.

In the initial time interval from time zero to time t capacitor C21 isrepresented as being charged from some arbitrary initial value such aszero up to its maximum negative potential as determined by the voltageof zener diode D30. The charging path is from the minus 50 voltconductor 47 through contacts KS-C and conductor 60 to capacitor C21 andthen to the common return conductor 50. At this time relay K5 isenergized (for example as a result of actuation of the start button toclose contacts Start B" at the lower part of FIG. lb). Relay K3 will notbe energized until the press reaches operating speed, so that prior tothis time relay contacts K3-C are closed, disabling the regulator 32 andmaintaining the output potential at zero as represented by the curvesegment 91 in FIG. 3.

When the start button is released, relay K5 is deenergized, closingcontacts KS-A at the upper right of FIG. lb to enable the regulatingcircuit including conductor and transistor Q20 which is controlled bythe charge on capacitor C21. Once the system has reached operatingspeed, regulator 32 will supply energizing current via conductor 31, tothe high-voltage generator 30, with the output voltage being controlledby means of the feedback line 34 to comparator Q4.

Contacts KS-B at the lower left in FIG. lb, while closed, preventconduction of silicon-controlled rectifier Q19, and Q19 remainsnonconducting when relay K5 is deenergized. Accordingly, capacitor C21has an efiective discharge path to line 46 through D24 and R49, anddischarges relatively rapidly as indicated at curve 82, FIG. 2. Theoutput potential correspondingly increases as indicated at 92, FIG. 3.The rate of discharge of capacitor C21 may be relatively rapid, forexample, corresponding to an output potential rise at 92 of the order of2,000 volts per second.

When the output potential applied to the substrate 12 reaches a limitingpotential value approaching the breakdown potential of the substrate asindicated at 920, FIG. 3, the current in potentiometer P4 is such as toautomatically trigger the crowbar amplifier Q3, Q7, and initiate a powerinterrupt cycle by rendering thyratron Q16 conductive. With energizingcoil 101 of relay K1 energized, contacts Kla are closed, disablingregulator 32 and turning off high-voltage generator 30. At this time,current through potentiometer P4 is essentially zero, to restorethyratron Q16 to its its nonconducting condition. The negative pulsegenerated by opening and closing of contacts Klb of relay Kl results inthe transmission of a negative-going pulse via capacitor C6, conductor38, resistor R40 and diode D27 to render transistor Q17 momentarilyconductive. This in turn renders silicon-controlled rectifier Q19conductive, and adds a predetermined increment of charge to capacitorC21 (as indicated at 83 in-FIG. 2) by virtue of the momentary conductionoftransistor Q18.

The output potential now builds up to a reduced value as indicated at940 which may be approximately [0 percent less than the limiting valueas indicated at 92a in FIG. 3. The charge on capacitor C21 now leaks offat a greatly reduced rate as indicated at 84, FIG. 2, allowing theoutput potential to build up gradually as indicated by ramp waveformsection 94 in FIG. 3.

When a limiting value as indicated at 94b, FIG. 3, is reached, a furtherpower interrupt cycle ensues with the output potential being reduced toa value such as indicated at 960 at the end of the power interruptcycle. The value 96a may be about 10 percent less than the limitingvalue 94b, and the output potential may again build up very gradually byvirtue of leakage from capacitor C21 as represented by curve 86, FIG. 2.

If a succession of power interrupt cycles should be encountered asrepresented at 87 in FIG. 2, thyratron Q16 will become conductive, andregulator 32 will be held off during the conduction of siliconcontrolled rectifier 01 as determined by the timing cycle of transistorQ12. This safety cycle is initiated when capacitor C3 acquiressufficient charge to cause neon tube N5 to become conducting.

While silicon-controlled rectifier O8 is nonconducting (during thetiming cycle ofQl2), transistor Q18 is held conducting to allow chargingof capacitor C21 to the maximum negative value as indicated at curve 88,FIG, 2. When Q8 becomes conducting, 019 is commutated to nonconductingcondition, allowing capacitor C21 to be relatively rapidly dischargedthrough D24 and R49 as indicated by curve 89, the output potentialrising rapidly as indicated by curve 99 until it again reaches theneighborhood of the limiting potential value 99a, FIG. 3.

It the dash line 105 through the successive points of limiting potentialsuch as 92a, 94b, and 96b represents essentially the variation ofbreakdown potential during a normal operating cycle, it will beobservedthat the operating potential is maintained essentially between thislimit and the reduced values such as 940 and 960, so that essentiallytheoperating potential applied to the substrate during normal operationfollows the dielectric breakdown strength of the substrate and ismaintained sufiiciently near the limiting potential value so as tomaintain substantially optimum transfer of ink to the substrate duringnormal operating conditions.

ILLUSTRATIVE PARAMETERS FOR THE PREFERRED CIRCUIT The following are thepreferred parameters for the circuit illustrated in FIGS. 1a and 1bwhich circuit has been built and successfully operated. (All resistorsare one-half watt with a precision of plus or minus 5 percent unlessotherwise specified. All capacitor values are given in microfarads witha rating of 100 volts unless otherwise specified.)

C18, 0.00047 kilovolts); C17, 0.00047 (10 kilovolts); R10, 22 ohms (2watts); C7, 10 (250 volts); C20, 10 (250 volts); R48, 1 megohm; D5, D33,D22each type 1N207l; R1, 2 ohms 12 watts); C1, 1,000 (50 volts); R3, 22kilohms; R4, 10 kilohms; R6, 10 kilohms; R9, kilohms; D1, D2-each typeMDA 970-2; C4, (50 volts); R7, 47 ohms; C3, 0.1; P1, zero to l megohm;D3, type lN459; C6, 0.1; C5, 0.22; R2, 10 ohms (2 watts); C2, 1,000 (50volts); Q1, type C6A; R5 220 ohms; C24, 0.001; R12, 4.7 kilohms 1 watt);D8, type 1N645; D9, type 1N34A; R19, 1.8 ki1ohms(l watt); 06, type2N3055; 05, type 2N2l02;Q2, type 2N699; R13, 15 kilohms; kilohms; D10,type 1N751A; R14, 47 kilohms; D4, type 1N645; C9, 0.05 (50 volts); C10,50 (50 volts); Q4, type 2Nl893; D6, type 1N4l56; Q7, type 2N3645; D7,type 1N459; R16, 100 kilohms; R15, '15 kilohms; C8, 220 picofarads(1,000 volts); R11, 15 kilohms; Q8, type C61; 012, type 2N2646; R22, 47ohms; R8, 47 kilohms; N10, type MB-2l6; R18, 1.8 kilohms (2 watts); R40,10 kilohms; R25, 22 ohms (10 percent); C13, 50 volts); P9 zero to 3kilohms; 020, type 2N4221; R50, 10 megohns; D26, type 1N64S; R51, 10kilohms (1 watt); C23, 1 (35 volts); D27, type 1N459; Q17, type 2N4249;03, type 2N4249; R23, 220 kilohms, C22, 10(50 volts); R41, 10 kilohms;C19, 0.22; D20, type 1N400l; R43, 1 megohm; D19, type 1N4l56; R42, 12kilohms; R21, 100 kilohms; 019, type C61"; R44, 220 ohms; D31, type1N645; P10, zero to 20 kilohms (one-quarter watt); 22 megohms; D24, type1N3595; C21, 5; D25, type 1N459; R46, 10 kilohms; R45, 47 kilohms; R47,47 megohms; Q18,

10 type 2N38 59A; D21, type iu'sss's; D29, type 1N3595; 030, type1N748A; R52, 15 kilohms; R53, 4.7 kilohms 1 watt);

D12, type 7715-6; D13, type 7715-6; ()9, type 2N3583; Q10,

type 2N3440; D11, type 1N645; Q15, type 2N5322; R28, 47 kilohms (1watt); C15, 0.05; R26, 56 kilohms; R17, 6.2 kilohms; C16, 0.05; R24, 56kilohms; R20, 4.7 kilohms (1 watt); P2, zero to 5 kilohms; D15, type7715-6; D14, type 7715-6; T3, 134-181; Q14, type 2N3583; Q13, type2N3440; R27, 6.2 kilohms; P4, zero to 10 kilohms; C11, 0.05; D16, type1N645; Q11, type 2N5322; C14, 0.00047, (10 kilovolts); P5, zero to 1kilohm; R30, 1.8 megohm (2 watts, 1 percent); R31, 2 megohms (2 watts 1percent); R32, 2 megohms (2 watts 1 percent); P7, zero to kilohms; P6,zero to 3 kilohms; D18, type 7715-6; D17, type 7715-6; R33, 240 kilohms(2 watts 5 percent); R34, 240 kilohms (2 watts 5 percent); R35, 240kilohms (2 watts 5 percent); R36, 240 kilohms (2 watts 5 percent); R37,240 kilohms (2 watts 5 percent); R38, 240 kilohms (2 watts 5 percent);R39, 240

kilohms (2 watts 5 percent); Q16, type 5557.

I claim as my invention:

1. In a printing system, an electric circuit having an output forsupplying an electric potential across an ink-receiving substrate forassisting in the transfer of ink to the substrate, said circuitcomprising electrical energy supplying means for supplying said outputelectric potential, automatic control means controlling said electricalenergy supply means and operable during normal operation for graduallyincreasing said output electric potential at a relatively gradual rateof increase, auto matic sensing means for automatically sensing when theoutput electric potential reaches a limiting potential value substantially equal to the breakdown potential of the substrate and forsignaling such limiting potential condition, and automatic setback meanscoupled to said sensing means and responsive to said limiting potentialcondition during normal operation for automatically reducing the outputelectric potential to a reduced magnitude which is less than saidlimiting potential value but which is of a magnitude to maintaintransfer of ink to the substrate, said automatic control means beingautomatically operable to gradually increase the output electricpotential from said reduced magnitude at the completion of each cycle ofthe automatic setback means during normal operation, and safety circuitmeans responsive to a severe fault at the substrate to substantiallyremove the electric output potential during a safety cycle and to resumeoperation with a minimum value of electric potential which issubstantially less than said reduced magnitude, said automatic controlmeans being operable after a safety cycle to increase the outputelectric potential at a relatively rapid rate substantially greater thanthe relatively gradual rate of increase during normal operation, andsaid automatic control means being operable in response to the outputpotential reaching the limiting potential value to resume normaloperation in the absence of a further severe fault in the substrate.

it P t I? t

1. In a printing system, an electric circuit having an output forsupplying an electric potential across an ink-receiving substrate forassisting in the transfer of ink to the substrate, said circuitcomprising, electrical energy supplying means for supplying said outputelectric potential, automatic control means controlling said electricalenergy supply means and operable during normal operation for graduallyincreasing said output electric potential at a relatively gradual rateof increase, automatic sensing means for automatically sensing when theoutput electric potential reaches a limiting potential valuesubstantially equal to the breakdown potential of the substrate and forsignaling such limiting potential condition, and automatic setback meanscoupled to said sensing means and responsive to said limiting potentialcondition during normal operation for automatically reducing the outputelectric potential to a reduced magnitude which is less than saidlimiting potential value but which is of a magnitude to maintaintransfer of ink to the substrate, said automatic control means beingautomatically operable to gradually increase the output electricpotential from said reduced magnitude at the completion of each cycle ofthe automatic setback means during normal operation, and safety circuitmeans responsive to a severe fault at the substrate to substantiallyremove the electric output potential during a safety cycle and to resumeoperation with a minimum value of electric potential which issubstantially less than said reduced magnitude, said automatic controlmeans being operable after a safety cycle to increase the outputelectric potential at a relatively rapid rate substantially greater thanthe relatively gradual rate of increase during normal operation, andsaid automatic control means being operable in response to the outputpotential reaching the limiting potential value to resume normaloperation in the absence of a further severe fault in the substrate.